A polymer electrolyte fuel cell usually has an air electrode provided at one side of an electrolyte and a fuel electrode provided at the other side of the electrolyte. The air electrode and the fuel electrode are both formed from a conductive porous body with a catalyst. In general, a plurality of such structures is stacked together via separators to form a polymer electrolyte fuel cell. The air electrodes and the first separators are in contact with each other, and the fuel electrodes and the second separators are in contact with each other. Contact resistance therebetween should be low. The separator is commonly a carbon plate or a metal plate. Examples of the conductive porous body for forming the air electrode and the fuel electrode include nonwoven fabric of carbon fiber which is called carbon paper, and porous metal, and so on. If a metal plate, such as a titanium plate, is employed as the separator for the polymer electrolyte fuel cell, the titanium plate is plated with Au in order to reduce contact resistance that may be high in a use environment due to an oxide layer having high electrical resistance formed on the titanium surface. Such an Au-plated titanium plate is heat-treated to reduce the contact resistance (see Patent Document 1).
A known method of forming an Au coating on the surface of the titanium plate includes depositing Au after removing a Ti oxide layer formed on the titanium plate (see Patent Document 2).
It has been considered to employ porous titanium having great corrosion resistance as a conductive porous body for the air electrode and the fuel electrode of the polymer electrolyte fuel cell. Porous titanium usually includes continuous holes 1 opening on the surface and being connected to inner holes and a skeleton 2, as shown in FIG. 7. An enlarged view of area A of the skeleton 2 of the porous titanium in FIG. 7 is shown in FIG. 8. As shown in FIG. 8, a Ti oxide layer 3 is naturally developed on a surface of the skeleton 2 of the porous titanium when the porous titanium is left in the atmosphere. In particular the Ti oxide layer 3 formed on an outer skeletal surface 4 of the porous titanium is known to lower conductivity and thus increase contact resistance. When the porous titanium is used for the air electrode and the fuel electrode of the polymer electrolyte fuel cell, it is therefore preferable to employ the porous titanium having Au coating on the surface as in the separator.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2004-134276
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2001-6713